This invention relates to preparation of low molecular weight alkali soluble polymers by aqueous suspension polymerization. In particular, it relates to preparation of homogeneous, acid-containing, alkali-soluble copolymers in discrete polymer droplets under steady state conditions in an aqueous suspension of a polar cosolvent.
It has long been desired to prepare low molecular weight, acid-containing polymers in a low pressure, conventional reactor, which exhibit a uniformity of composition normally associated with polymers prepared in solution. Solution polymerization techniques employ large quantities of expensive solvents, which must be recovered by high-temperature stripping operations for economy, which operations may create safety problems. In addition, until now, it has proven difficult to prepare a homogeneous distribution of a low molecular weight copolymer product, utilizing solution polymerization techniques, of water soluble and water insoluble monomers.
Monoalkenyl aromatic and acrylic addition copolymers of number average molecular weight (M.sub.n) from about 1000 to 5000 and weight average molecular weight (M.sub.w) from 3000 to 8000 are often designated "solution polymers". Such acid-containing, alkali soluble copolymers are widely utilized in inks, floor finishes, paper coatings, paints and adhesives. For commercial use, these copolymers are often prepared in concentrates of high solids content. Often they are converted to water soluble "resin cuts" by reaction with a suitable base. To maintain satisfactory low viscosity, clarity and color, solution polymers should exhibit a relatively uniform composition and a low polydispersity index (M.sub.w)/(M.sub.n), usually less than about two.
Heretofore, aqueous suspension systems were unsuitable for preparation of "solution polymers" of homogeneous composition and narrow molecular weight distribution adapted to yield clear, non-viscous ammoniacal solutions. Acrylic acid and other water soluble monomers normally employed in solution polymers partition strongly in the aqueous phase and tend to form an acrylic homopolymer, either as a shell around the water insoluble monomer/copolymer particles or as a discrete particle.
To overcome this problem, an approach was developed which promoted the solubility of acrylic acid monomer in the water insoluble comonomer (styrene) phase to yield a higher acrylic acid content copolymer. This approach utilized concentrated salt solutions to reduce the solubility of acrylic acid in the aqueous phase. This route did not succeed owing to the adverse effects on surface and interfacial tension created by the use of concentrated salt solutions, as set forth in U.S. Pat. No. 3,862,924.
Previous researchers studying emulsion polymerization of copolymer resins have reported that significant polymeric initiation takes place in the aqueous phase or on the latex' surface, rather than in the latex' core site, see Jour. Appl. Poly. Sci., Vol. 23, 893-901 (1979) and Jour. Appl. Poly. Sci., Vol. 20, 2583-2587 (1976). Accordingly, emulsion polymerization techniques do not permit preparation of homogeneous copolymers of water soluble and insoluble monomers.